1. Field of the Invention
The present invention relates to a scanning optical system.
2. Description of Related Art
A scanning optical system is essential to the operation of a laser printer, a laser scanner, a bar code reader or the like. In a typical scanning optical system, a polygonal mirror, a hologram disk or the like is used as a light deflector. A laser beam emitted from a semiconductor laser is incident upon, and deflected by, the light deflector. The laser beam subsequently passes through an f.theta. lens system which acts as a scanning lens, to scan a predetermined area on a scanning surface, i.e., the main scanning is executed. The f.theta. lens system enables the beam spot to move at constant speed along the scanning surface. The scanning surface could be, for example, a sensitive paper or plate. While the main scanning is being executed, the scanning surface is moved in a direction perpendicular to the direction of the main scanning, i.e., sub-scanning is executed. Thereby, the scanning surface is scanned two-dimensionally.
Regarding the f.theta. lens system, there have been a number of ideas proposed for correcting the curvature of field, chiefly in the main scanning direction. However, all of the ideas have adopted a common construction in which a plurality of lenses and/or an aspherical surface formed on a lens is utilized to make up the f.theta. lens system for correcting the curvature of field in the main scanning direction. However, this results in a high production cost, especially when many lenses are necessary for the correction. Furthermore, forming an aspherical surface on a lens is a difficult process.
In order to correct the curvature of field in the main scanning direction with an f.theta. lens system consisting of a plurality of lenses, it is generally considered necessary for at least one surface of a lens in the f.theta. lens system, such as a toric surface, to be aspherical in a main scanning plane. However, forming a toric surface as an aspherical toric surface is a difficult process.
Yet in the f.theta. lens system, although there have been a number of proposals for correcting the curvature of field in the main scanning direction, as noted before, there have been no proposals, as of yet, for effectively correcting the curvature of field in the sub-scanning direction. That is, little attention has been paid to the occurrence and effect of the curvature of field in the sub-scanning direction. This is partly because it is difficult to correct the curvature of field in the sub-scanning direction with an f.theta. lens system consisting of a small number of lenses. Of course, the f.theta. lens system could be made up of a small number of lenses by forming a complicated surface on one of the lenses in the system. However, such f.theta. lens system is not suitable for mass production.